Cell-to-cell interactions between different genera of oral bacteria (coaggregation interbacterial adherence) are considered of cardinal importance in the formation and persistence of tooth surface plaque. Studies on coaggregation between Actinomyces (coaggregation groups A-F) and oral streptococci (coaggregation groups 1-6) have identified 12 specific interaction patterns. The patterns are determined by the effect on coaggregation of heating or protease treatment of bacteria and after the addition of lactose to a coaggregating pair. The majority of the coaggregations identified are inhibited by lactose. The present study examined a lactose-inhibitable coaggregation that is highly specific in that it occurs between Propionibacterium acnes and only coaggregation group 1 streptococci; e.g. Streptococcus sangius DL1. Large, stable coaggregates formed and settled rapidly. Coaggregation was inhibited by lactose; of 10 galactosides tested, n-acetylglactosamine was the best inhibitor. Heating of the S. sanguis cells, but not the P. acnes cells, destroyed coaggregation ability. Coaggregation occurred in the presence of human saliva and was also inhibited by lactose. Saliva-induced aggregates of bacteria were easily differentiated from coaggregates by differences in size, stability and inhibition by lactose. In saliva, coaggregates formed at the expense of aggregates. Interbacterial adherence of P. acnes and S. sanguis also occurred on a saliva-coated surface. The attachment of the bacteria to the surface was not affected by lactose whereas the interbacterial adherence was inhibited. Similar results were obtained with saliva present in the liquid phase of the assay system. These findings further support the contention that very specific interactions involving lectins occur in vivo between different genera of oral bacteria and lead to the formation of unique bacterial compositions of supragingival and subgingival plaques.